The invention relates to a gas discharge lamp, in particular a low-pressure gas discharge lamp, which comprises an electrode including a carrier and a coating of an electron-emitting material, which material comprises an alkaline earth metal oxide, which is selected from the group formed by calcium oxide, strontium oxide and barium oxide, and an oxide of a rare earth metal.
The generation of light in a gas discharge lamp is based on the ionization, and the resulting electric discharge, of the atoms of the filling gas in the lamp when an electric current flows through the lamp. The electrodes of the lamp emit electrons, which are so strongly accelerated by the electric field between the electrodes that, upon colliding with the gas atoms, they are capable of exciting and ionizing the latter. When the gas atoms return to the ground state, and also in the case of the recombination of electrons and ions, a more or less substantial part of the potential energy is converted to radiation.
The number of electrons that can be emitted by the electrodes depends upon the work function of the electrodes for electrons. Tungsten, which is customarily used as the electrode metal, has a comparatively high work function. For this reason, the electrode metal is customarily coated with a material which serves to improve the electron-emitting properties of the electrode metal. It is typical of the electron-emitting coating materials of electrodes in gas discharge lamps that they contain an alkaline earth metal, either in the form of the alkaline earth metal oxide or in the form of an alkaline earth metal-containing starting compound (precursor) for the alkaline earth metal oxide.
Thus, conventional low-pressure gas discharge lamps are generally provided with electrodes which are composed of tungsten wires with an electron-emitting coating containing oxides of the alkaline earth metals calcium, strontium and barium.
To manufacture such an electrode, a tungsten wire is coated, for example, with the carbonates of the alkaline earth metals in a binder preparation. During evacuating and baking out the lamp, the carbonates are converted into the oxides at temperatures of approximately 1000xc2x0 C. After this xe2x80x9cburn-offxe2x80x9d of the electrode, the electrode already supplies a noticeable emission current which, however, is not stable yet. Next, an activation process is carried out. Due to this activation process, the originally non-conducting ion lattice of the alkaline earth oxides is converted to an electronic semiconductor. In this process, donor-type imperfections are incorporated into the crystal lattice of the oxides. These lattice imperfections essentially consist of elementary alkaline earth metal, for example calcium, strontium or barium. The electron emission of such electrodes is based on this mechanism of lattice imperfections. The activation process serves to provide a sufficient quantity of excess, elementary alkaline earth metal, enabling the oxides in the electron-emitting coating to supply as much emission current as possible at a prescribed heating capacity.
As regards the function of these electrodes and the service life of the lamp, it is important that elementary alkaline earth metal is constantly available. The reason for this being that the electrode coating continuously loses alkaline earth metal during the service life of the lamp, which is partly caused by the fact that the electrode coating evaporates slowly, and partly by the fact that the electrode coating is sputtered off by the ionic current in the lamp.
The elementary alkaline earth metal is initially dispensed continuously by a reduction of the alkaline earth oxide at the tungsten wire during operation of the lamp. However, this dispensation stops when the tungsten wire is passivated, in the course of time, by a highly resistive interface of tungsten oxide, alkaline earth silicate or alkaline earth tungstate.
DE 1 021 482 discloses a method of manufacturing an oxide cathode for low-pressure discharge lamps, the activating substance of which is composed of a mixture of barium oxide, strontium oxide and calcium oxide, which are formed during the activation of the cathode by thermal decomposition of the alkaline earth carbonates used as the starting material, an inactive additive composed of at least one oxide of the elements: titanium, germanium, aluminum and other elements of group III of the periodic system, particularly the rare earth elements, being added to the alkaline earth-carbonate mixture in such a quantity that the overall quantity of the added oxides in the completely activated cathode does not exceed the smallest quantity of alkaline earth oxide used, and the cathode is activated by heating to a temperature below 1,000xc2x0 C., preferably 800xc2x0 C. to 900xc2x0 C. This method has the advantage that the carbonates are rapidly decomposed at low temperatures and the lamp does not contain carbonic acid.
It is an object of the present invention to provide a gas discharge lamp with an extended service life and an improved emission current.
In accordance with the invention, this object is achieved by a gas discharge lamp comprising an electrode including a carrier of an electrode metal, selected from the group formed by tungsten and tungsten-containing alloys, and a first coating of a first electron-emitting material, which material comprises an alkaline earth metal oxide, selected from the group formed by calcium oxide, strontium oxide and barium oxide, and a rare earth metal oxide selected from the group formed by scandium oxide, yttrium oxide and europium oxide in a quantity that ranges from 0.1 to 10 wt. % by weight.
In such a gas discharge lamp the passivation of the electrode metal is reduced, so that alkaline earth metal is released from the oxide over a longer period of time and the work function of the electrode remains low. This results in a shorter ignition stage of the lamp. At the same time, the addition of a rare earth metal oxide, in a quantity xe2x80x9cAxe2x80x9d that ranges from 0.1 to 10 wt. % by weight, brings about a reduction of the evaporation of elementary alkaline earth metal and hence leads to a longer service life. The electrode has a high initial emission and contains sufficient elementary alkaline earth metal throughout the service life of the lamp. The availability of sufficient elementary alkaline earth metal also leads to a high resistance to poisoning by oxygen.
These advantageous effects are enhanced if a second coating of a second electron-emitting material is arranged between the carrier and the first coating, said second electron-emitting material comprising an alkaline earth metal oxide, selected from the group formed by calcium oxide, strontium oxide and barium oxide, and a rare earth metal oxide, selected from the group formed by scandium oxide, yttrium oxide and europium oxide, in a quantity xe2x80x9cBxe2x80x9d that ranges from 2.0 to 20% by weight.
Particularly advantageous effects are achieved when quantity a less than quantity b.
It may also be preferred to provide a third coating between the carrier and the first coating, which third coating is composed of a noble metal selected from the group formed by rhenium, cobalt, nickel, ruthenium, palladium, rhodium, iridium, platinum. Such a gas discharge lamp has a reduced ignition stage, and the electrode accommodated in such a lamp has an improved conductivity.
It may further be preferred that the first electron-emitting material comprises zirconium oxide. It may also be preferred that the second electron-emitting material comprises zirconium oxide.
It may additionally be preferred that the first electron-emitting material comprises a metal powder preparation of a metal selected from the group formed by aluminum, silicon, titanium, zirconium, hafnium, tantalum, molybdenum, tungsten and the alloys thereof, which metal powder preparation is provided with a powder coating of a noble metal selected from the group formed by rhenium, cobalt, nickel, ruthenium, palladium, rhodium, iridium and platinum.